1. Technical Field
This invention relates to brake pad and back plate structures employed on vehicles or machinery, and more particularly, to integrated fiber reinforced ceramic matrix composite (FRCMC) brake pad and back plate structures and methods of integrally molding such structures.
2. Background Art
Any vehicle, such as an aircraft, truck, automobile, train, or the like that employs wheels typically is provided with a brake system with which to stop it. The lighter the combined stopping weight, the fewer the problems involved in designing a brake system which will last for an extended period of time and then be easy and inexpensive to replace or renovate. Thus, a vehicle such as a bicycle can be fitted with small rubber pads that squeeze and grip the rims of the wheels which will last virtually forever and which can be replaced in a few minutes at little expense.
When one gets to the mass of an automobile, which may contain a number of passengers, frictional heat build-up during stopping becomes a problem to be considered. Most automobiles today employ a so-called caliper disk brakes on at least the front wheels since during stopping the weight of the vehicle is moved forward to the front wheels due to the force of inertia. Disk brakes as depicted in FIG. 1 have good stopping power for various reasons. A rotor 10 carries the wheel (not shown) on a shaft 12. As the wheel rotates, the rotor 10 rotates in combination with it. The rotor 10 is disposed between a caliper 14 having opposing pistons 15 which respectively abut the back plates 17 of a pair of brake pad and back plate assemblies. The brake pad 16 of each assembly is positioned so as to face a respective surface of the rotor 10.
To stop the automobile, hydraulic pressure is typically used to move the pistons 15 together until the rotor 10 is squeezed under pressure between the pads 16. The caliper 14 is attached to the frame of the automobile and so prevents the pistons 15 and brake pad and back plate assemblies from rotating when the pads 16 are pressed against the rotor 10. The pads 16 are made of a high friction material that resists deterioration and wear under fairly high temperature conditions. Thus, when the rotor 10 is squeezed by the brake pads 16, a high frictional stopping force is applied to the rotor 10, bringing the automobile to a stop. Since the pads 16 are flat and contact the flat sides of the rotor 10, the entire area of the pads 16 contacts the rotor 10 to impart the stopping forces. While the brake pads 16 impart the frictional stopping force, the back plate 17 is designed to withstand the considerable compression and bending forces placed on it by the piston 14 when the brake pads are held against the rotor 10. FIG. 2 illustrates the forces placed on the back plate 17 during braking. Typically, a disk brake piston 15 has an annular or circular face 18 that contacts a small area on the back plate 17 generally near the center of the plate's elongated surface. The back plate 17 must first withstand the compressive force (F.sub.C) placed on it when the piston 15 is extended to push the brake pad against the rotor. Additionally, it must resist bending under the bending force (F.sub.B). This bending force is created when the piston 15 is extended because the back plate 17 is typically restrained at its ends 20 and because the piston 17 pushes against only a small area in the center of the back plate. Should the back plate 17 bend, the brake pad, which is typically brittle in nature, could fracture and fail. Thus, the back plate 17 must be tough and stiff in its longitudinal direction to withstand the compressive and bending forces encountered. The back plate 17 is usually made of metal, typically steel, and has a thickness sufficient to withstand the compressive and bending forces associated with the braking application. For example, a metal back plate 17 s used in brake pad and back plate assemblies on automobiles is typically between about 0.15 and 0.20 inches thick.
Typically, brake pads are riveted to the back plate to form the brake pad and back plate assembly. However, a common problem with this type of assembly is the incidence of the rivets "scoring" the brake rotors when the brake pads wear down too far past the point of needing replacement. More recently with the advent of high strength adhesives, so-called "bonded" brake pads have been employed wherein the pads are attached with adhesive to back plate. With the bonded brake pads and back plate assemblies, the pads can wear to the point that the back plate itself is contacting the drum or rotor before damage can occur. Unfortunately, the use of adhesives to bond the pad to the back plate presents problems of its own. First, the types of high strength adhesives appropriate for use in bonded brake pad assemblies are expensive and tend to be environmentally hazardous. Thus, their use can cause considerable difficulties that increase the costs of the brake pad and back plate assembly, or their use may be prohibited altogether by local environmental protection laws. In addition, the build up of heat in the brake pad and back plate assembly during braking operations can be extreme enough in some systems to burn or degrade many of the aforementioned adhesives. This can lead to detachment of the brake pad and cause the brake to fail. Further, there is typically a large difference in coefficients of thermal expansion between a brake pad made of FRCMC material and a metal back plate structure. This mismatch can cause the adhesive bond between the structures to fail when the bonded brake assembly heats up during braking, thereby also potentially leading to detachment of the brake pad and failure of the brake. The difference in expansion coefficients is particularly acute when a steel back plate is employed (as typically would be the case). In fact, to date the inventors of the present invention have been unable to find an adhesive capable of withstand the mismatch in expansion rate between an FRCMC brake pad and a steel back plate.
Another approach to attaching the brake pad to the back plate was described in a co-pending application entitled BRAKE ROTORS/DRUMS AND BRAKE PADS PARTICULARLY ADAPTED FOR MOTORIZED VEHICLES having the same inventors as the present application and assigned to a common assignee. This co-pending application was filed on Apr. 4, 1996 and assigned Ser. No. 08/627,398 now U.S. Pat. No. 5,806,636. The disclosure of this co-pending application is herein incorporated by reference. The previously disclosed approach involved casting a metal back plate onto a FRCMC brake pad. The FRCMC brake pad was employed because of its capability to withstand high temperature such as those encountered during braking of even large vehicles and machines, as well as being tailorable to provide a desired coefficient of friction. As shown in FIG. 3, FRCMC pads 16" are provided with gripping members 30 extending from the rear thereof. Since the FRCMC pads 16" are able to withstand extremes of temperature including that of molten metal, the back plate 17" can be cast onto the pad(s) 16". Finished pads 16" are placed within a mold 34 used to form the assembly. Molten metal 32 is then poured into the mold 34 and allowed to harden to form the brake pad and back plate assembly. However, even though this casting method worked well, it has been found to be relatively expensive and cost prohibitive for mass production.
Accordingly, there is a need for brake pad and back plate assembly that does not require the use of rivets or adhesives to attach the brake pad to the back plate, thereby avoiding the problems associated with each attachment system. Further, there is a need for a brake pad and back plate that employs a brake pad having the high-temperature and erosion resistance capabilities of FRCMC materials while avoiding the prohibitive costs associated with previously-described cast metal back plate design.
Wherefore, it is an object of the present invention to provide a brake pad and back plate which is integrated in that the brake pad and back plate are formed together as a unitary structure, without the need for rivets, adhesive or casting of metal to attach the brake pad to the back plate.
Wherefore, it is another object of the present invention to provide a brake pad and back plate that is entirely made from FRCMC materials, thereby providing resistance to high temperatures, as well as similar coefficients of expansion.
Wherefore, it is yet another object of the present invention to provide a brake pad and back plate wherein the FRCMC material making up the brake pad is tailored to provide a prescribed coefficient of friction.
Wherefore, it is still another object of the present invention to provide a brake pad and back plate wherein the FRCMC material making up the back plate section of the assembly is chosen so as to provide an enhanced strength capable of withstanding the compressive and bending forces imposed during braking, as well as being lighter in weight than traditional metal back plates.